1. Field of the Invention
The field of this invention lies within the structural field as it relates to joints, couplings, bolts, and various fastening means. More particularly, it involves the field of providing greater strength within a joint by means of pre-loading and pre-stressing through a supplemental structural member extrinsic to the basic member itself.
1. The Prior Art
The prior art related to joints, fasteners, bolts, and other structural components for such things as pipes, oil well casings, and similar shell structures, has generally involved many types of joint and fastener configurations. Some of these have incorporated bolt and nut configurations with lock washers and other fasteners.
In addition to the foregoing fastening means, shell structures have incorporated various tensioning and clamping devices, as well as tension wrap means and other structures for securing a joint. Oftentimes, such shell structures are threaded together with heavy duty threads, so that the elements to be joined can be torqued down on each other to provide a substantial degree of tightness and fit.
In most designs, the weakest point of a member that uses the inherent structure of the member for coupling purposes is within the joint. In other words, when a tubular member is threaded together or joined, a substantial problem resides within the threaded joint. This is due to the fact that certain frequency and vibratory modes of the structure or tubular member limit the design functions.
A design requirement is to increase the joint stiffness to the same degree of the entire structure. In other words, the tubular member as it exists without a joint has a certain given stiffness. If the joints can also be maximized to the same degree of stiffness to retain a stiff joint, a substantial reduction of overall shell thickness or structural weight can be reduced. It has been found that the structure of such items can be reduced as to overall shell weight by increasing the joint stiffness by a significant factor. Other configurations, such as pipes for oil wells, casings, shell structures and pipelines that span chasms are subject to a bending moment, as well as other joints and configurations which have stiffness requirements.
One of the principal requirements for a high-stiffness joint is the application of a large pre-load. This can be developed in flange joints with threaded fasteners. However, for certain applications, this type of joint generally has unacceptable volume protrusion and accessability.
Threaded and breach lock joints are difficult to pre-load because of friction within the threads. Furthermore, they are susceptible to unloading during vibratory or shock conditions. In other words, even though a threaded joint within a tube, pipe, or other shell can be loaded substantially, it must be remembered that the threads have a certain coefficient of friction between the lands and the grooves and as the friction builds up during the tightening process, it eventually limits the pre-load that can be established by the friction of the threads.
In addition to the foregoing problems, a problem has evolved with regard to the engine head bolts or other bolted elements. This is particularly true with respect to maintaining head bolts on diesel engines which have a tremendous amount of vibration. The head bolts, as can be understood, in any internal combustion engine, must be torqued to a certain point to maintain both compression and a seal. In many cases, the head bolts on such internal combustion engines are placed in a vibratory mode and eventually loosened to the point where the original torque is no longer present. This, of course, creates problems with regard to a lack of a seal, as well as the bolt jarring loose at a later point in time.
By utilizing the shape recovery characteristics of Nitinol, which is fundamentally a nickel titanium alloy, which can also be alloyed with cobalt for providing lower temperatures of transformation, a pre-loaded structural joint can be designed in various configurations. The basic design criteria for the pre-loaded structural joint is such that its strength or stiffness should be equal to or greater than that of the surrounding shell or other structure over its full load range. This also applies to the fundamental usage of the structure herein with regard to head bolts or other structural tightening means. It should also provide resistance to vibratory unlocking.
Many of these basic design criteria are incorporated to provide the foregoing enhanced features for overcoming the deficiencies of the prior art, as previously stated. In addition thereto, the Nitinol pre-loaded joints of this invention are a substantial step over the prior art as to the evolvement and development of pre-loaded structures and in particular, structures that retain a desired value of stiffness.